problematic view definition

Hi all !

Attached find some table and view definitions from the
GNUmed (www.gnumed.de) database.

Unfortunately I do not understand why PostgreSQL says

    psql:xx.sql:14: ERROR:  could not implement UNION
    DETAIL:  Some of the datatypes only support hashing, while others only support sorting.

when I say

    select * from dem.v_message_inbox;

I mean, I (hope I) do understand what PostgreSQL tries to
tell me but I don't know how to find out which columns are
affected ...

Thanks !

Karsten
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